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When worms find fault!

03 September 2009

Allen Joyce - Training and technical manager

Unlikely as it may seem, worms can be a big help in accurately pinpointing the location of faults in buried cables. It has been reported that two groups of worm – annelids and aschelminths – are the most useful, but for those cable test technicians who are possibly a little lacking in worm expertise, it will probably be a relief to know any healthy ground worms will do.

The basis for cable fault locating with worms is the feeding habits of birds, many of which have a strong liking for eating fresh healthy juicy worms. When they are hungry, these birds stamp on the ground. In response, the worms come to the surface where, of course, the birds eat them.

Cable fault location using worms has many similarities except the engineer performing the test is not actually required to eat the worms unless, of course, they really want to! This is how the technique works.

Once the fault has been pre-located using conventional high or low voltage techniques, its precise location needs to be pinpointed so that it can be excavated, repaired and the cable put back into service. Most often, faults are pinpointed acoustically. This method involves using a high voltage pulse from a surge generator (sometimes called a thumper) to produce a flashover at the site of the fault. The flashover generates noise, which is detected using a ground microphone and an acoustic receiver. This method allows the location of the fault to be pinpointed very accurately.

So where do the worms come in?

The answer is that they can take the place of the ground microphone and acoustic receiver!

With the worm technique, a surge generator is still used to create the flashover but, instead of listening for the noise this creates, the test engineer looks for worms coming out of the ground. The worms respond to the noise from the flashover just as they would if a bird was tapping the ground – in other words, they rise to the surface.

This method is almost as accurate as the conventional technique. It also has the advantage of working best in ground that’s wet or boggy, conditions that can often hamper the pinpoint location of faults using standard acoustic methods.

Best of all, if you use this technique, you don’t even have to bring your own worms – they’re already there, in the ground, patiently waiting to help you to locate that elusive fault!

To see Megger's extensive range of cable fault equipment click here